The present invention relates to methods for testing weighing devices generally, and more particularly to methods for ensuring minimal error in weighing devices. The present invention further relates to methods for determining whether weighing device calibration is necessary.
Weighing devices are currently utilized in a wide variety of applications, including various commercial applications. In many of such commercial applications, the weighing devices are utilized to determine price or cost of bulk products based on a given cost per unit of weight. Examples of such commercial applications include truck scales for measuring payload weights, hanging scales, shipping scales, weight classifiers, and bench or counter scales for determining weights of products such as pharmaceuticals, produce, or any other bulk merchandise sold on a per unit weight basis.
In many cases, the weight displayed by the weight-measuring scale corresponds directly to the price of the product whose weight is being determined. The product weight determination is typically conducted on a single measurement basis, such that variances in measurement or scale characteristics may be exacerbated. Further, systematic errors in the weighing process can adversely affect the seller or buyer of the measured product, depending upon which direction away from accuracy the systematic error presents in the weighing process. For example, a weighing device consistently displaying a weight less than actual weight results in reduced sale price and, consequently, reduced profits for the seller. Therefore, errors, whether random or systematic, are desirably minimized to reduce negative economic impact to both the seller and the buyer.
In a particular application of product weighing in the commercial setting is in the sale of grocery items at the checkout counter. In such an application, grocery items sold by unit weight are weighed at the checkout counter to determine the item price. Scales utilized at such checkout counters are typically of sufficient size to accommodate relatively large items, such that smaller items placed anywhere on the scale must also be accurately measured for their weight.
As discussed above, weighing devices reflecting measurement error adversely affect the seller or buyer, depending upon the measurement direction from zero error on the load applied. This condition is particularly exacerbated when multiple weighing devices are utilized at a particular place of business where more weighing devices than not are consistently measuring in a particular direction from zero error. In some instances, sophisticated sellers alter their scales to consistently measure a weight greater than what is accurate, such that the price charged to the buyer is correspondingly higher than the true price of the item. To counter such practices, governmental standards have been developed to periodically test weighing devices in a wide variety of commercial applications. Governmental agencies have been created to enforce such standards. In the case of a grocery store, governmental inspectors periodically test respective checkout counter scales by performing set testing procedures. Where weighing devices are determined to be non-conforming with stated governmental standards, various penalties may be assessed to the commercial proprietor.
As a safeguard procedure to prevent such penalties from being incurred, some proprietors employ testing procedures to ensure that their scales remain in compliance with governmental procedures. Typically, such testing professionals utilize those testing procedures developed by governmental agencies to ensure a minimal level of compliance with corresponding governmental standards. An example of a resource for determining appropriate governmental standards and testing procedures may be found in the National Institute of Standards and Technology (NIST) Handbook 44, which is produced by the United States Department of Commerce. The NIST Handbook 44 details test procedures and compliance standards for a variety of commercial weighing devices. The test procedures enumerated in the NIST Handbook 44 are intended to represent methods for determining minimal compliance with stated governmental standards.
To maintain their weighing devices in compliance with governmental standards, as well as to maintain a degree of accuracy, proprietors commonly utilize the NIST Handbook 44 test procedures. Because such test procedures are intended to provide only a minimal degree of compliance with stated governmental standards, it is desired to develop and utilize a testing method for minimizing error in weighing devices, which method provides an increase level of accuracy over published governmental test procedures.
It is therefore a principle object of the present invention to provide a method for minimizing error in weight-measuring devices such as scales.
It is a further object of the present invention to provide a test method for minimizing error in grocery checkout counter scale devices.
It is another object of the present invention to provide a reliable method for minimizing error in Class III scales.
It is a still further object of the present invention to provide a method for minimizing error in weighing devices, which method meets or exceeds the requirements set forth in the NIST Handbook 44.
It is a yet further object of the present invention to provide a test method utilizing a known weight of one-quarter to one-half of the scale capacity, which method minimizes the error in such a scale.
It is a still further object of the present invention to provide a method for determining whether weighing device calibration is necessary.
It is a still further object of the present invention to provide a test method for minimizing error in weighing devices, which method utilizes a sum of the errors technique to enhance determination of weighting device error.
By means of the present invention, an easy and efficient technique for minimizing the error in weighing devices is provided. Such a technique is achieved by successively placing a known weight at a plurality of distinct locations on a weight-receiving surface of the weighing device and determining a measurement error associated with each test location. The individual measurement errors are then added together to create a summed error, which summed error is compared to a desired tolerance level for the weighing device. The multiplicity of testing locations, along with summation of individual measurement errors provides an accurate depiction of weight measurement characteristics of a particular weighing device. Through such a method, various corrective measures may be undertaken in cases where the summed error exceeds the desired tolerance level, such that weighing devices may be screened for need of corrective actions on a relatively frequent basis, and at little expense to the weighing device operator.
One embodiment of the testing method of the present invention includes setting the weighing device at a zero point, successively placing one or more test loads on the weighing device at a plurality of distinct testing positions located in about a peripheral two-thirds of a weight-receiving surface of the weighing device, the loads being weighed by the weighing device at discrete instances such that the testing positions are utilized individually to measure a selected load. The user then determines the weight error displayed by the weighing device at each of the testing positions, sums the distinct measured weight errors into a summed error, and subsequently compares the summed error to a desired tolerance level, such that weighing devices exhibiting summed errors in excess of the tolerance level may be identified as being in need of calibration. Preferably, the distinct testing positions are substantially equidistant from one another, and distributed evenly about the weight-receiving surface. A common test load is preferably utilized at each of the testing positions wherein the test load is one-fourth to one-half of the designated weight capacity of the weighing device, and most preferably is one-third of the designated weight capacity of the weighing device. In preferred embodiments, the test loads are successively placed at least four distinct testing positions to determine device error at a weight range above capacity, thereby increasing accuracy of the test method. In some embodiments, the testing positions are located between a center point of the weight-receiving surface and respective outer corners of the weight-receiving surface.
Instances in which the summed error of a particular scale exceeds the designated tolerance level, the scale tester preferably conducts one or more of the following steps to perform a corrective action upon the scale: a) leveling the scale with leveling means, b) cleaning the scale, particularly under the weight-receiving surface of the scale, c) visually inspecting or passing a thin tool between the weight-receiving surface and the housing of the scale to ensure that the scale is free from obstructions which could impede its operational functions, and d) repairing or replacing broken or missing elements of the scale. A preferred method for calibrating a scale exceeding the designated tolerance levels includes utilizing a 2000 division weight range to thereby obtain the desired tolerance level, though a variety of weigh ranges may be employed in the calibration sequence.